My experience with zstart pro on a 2t & core exp on a 4t was that it remained engaged until the engine revs dropped to around idle which was the opposite to what I thought was going to happen.

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Slipper clutches on the other hand can be setup to do what you're after, although they are more aimed towards super moto use. The one's in the gassers used to be brilliant (sutter ??)

Some can be annoying in the dirt though as the slipping may also mean the engine has trouble restarting when you accidentally stall out into a corner and want it to quickly bump start back into life after you accidentally lock up the rear brake etc.

Depending on the bike, you might even be able to try a cam with more overlap to bleed off some compression or taller gearing or even higher air pressure etc.

The rekluse is basically an anti-stall device. A properly setup one shouldnt affect how the motor performs the moment it goes above idle.

The Zstart Pro can have ball bearings taken out to raise the rpm level of engagement, effectively disengaging upon braking. 30 balls are std , they recommend removing 3 balls then trying the clutch again.